docs: def unique(self, *args):
          """
          Returns all unique values as a DataFrame. This is executing:
              SELECT
                  DISTINCT
                      <name_of_the_column_1>
                      , <name_of_the_column_2>
                      , <name_of_the_column_3>
                      ...
              FROM
                  <name_of_the_table>
  
          Parameters
          ----------
          *args: columns as strings
  
          Examples
          --------
          >>> from db import DemoDB
          >>> db = DemoDB()
          >>> db.tables.Track.unique("GenreId")
              GenreId
          0         1
          1         2
          2         3
          3         4
          4         5
          5         6
          6         7
          7         8
          8         9
          9        10
          10       11
          11       12
          12       13
          13       14
          14       15
          15       16
          16       17
          17       18
          18       19
          19       20
          20       21
          21       22
          22       23
          23       24
          24       25
          >>> len(db.tables.Track.unique("GenreId", "MediaTypeId"))
          38
          """
          q = self._query_templates['table']['unique'].format(columns=self._format_columns(args), schema=self.schema,
                                                              table=self.name)
  
          return pd.read_sql(q, self._con)